Acquiring fatigue crack growth rate (FCGR) data at low frequencies and near threshold values of a test specimen's stress intensity factor requires a significant investment in time and equipment. Generally, the total time required to conduct a test can be reduced by increasing the frequency of cyclical loading, but some situations require tests to be conducted at low frequencies. These low frequency tests can occupy a load frame for months. For over four decades, researchers have sought methods to test multiple specimens simultaneously as a means of obtaining test data faster, while performing the tests at an appropriate rate. As early as 1966, Robert E. Little of the University of Michigan proposed fatigue testing thin cantilevered coupons (0.84 mm) with a vibration table at either high frequency (approximately 1,000 Hz) or low frequency (28 Hz). As many as 40 specimens could be tested simultaneously in order to generate statistics to predict lifetimes. This was a simple pass/fail test, in which the specimens were fatigued for 106 cycles and inspected for the presence of cracks.
Fatigue testing in bending was also applied to multiple specimens of bone cement, polymer, and composite. A technique proposed by Kim et al. rotates thin flat specimens with dead weights of varying magnitude on each specimen to generate S-N (Stress versus Cycles-to-failure) curves. The cycle at which a given specimen fails is noted and the test is stopped after 106 cycles. For biological materials an immersion bath could be used.
Similarly, apparatuses described by Imig and Garrett and that of Slota and Wegman were designed to generate S-N curves, as the crack length is not monitored during the course of the fatiguing. Both techniques, however, are designed to test nonstandard specimens in tension. The “specimen” of Imig and Garrett is a titanium alloy (57.2 mm wide by 1220 mm long) studied for use in supersonic transport airplanes with stress concentrations machined into the test sections at specific intervals. After one of the test sections fails, that section is removed, the sheet spliced (bolted) back together, and the test resumed. Their system imposes very complex loading sequences to mimic real-time in-flight conditions. Furthermore, thermal loads and corrosive environments could be accommodated. Slota and Wegman offered no dimensions for their specimen, but it appears to be a flat bar. Further description was limited to an actuation method, which employed air pistons to load each specimen in tension, and an electrical switching mechanism that cyclically activated air pistons.
Ermi et al. in 1981 proposed linking eight miniaturized center-cracked tension specimens and loading them cyclically in tension. The load was applied by cyclically pressurizing helium in a chamber via a bellows at the top of the chain of specimens. The initial fatigue pre-crack (FPC) was varied to generate different starting values of the range of the stress intensity factor (ΔK). The linked specimens were fatigued for a pre-set number of cycles, the crack lengths measured, and fatigue crack propagation (da/dN) curves were compiled from data from all the specimens. Elevated temperature tests were possible with this test system.
Multiple specimen fatigue testing with crack growth monitoring is even less common. Hartt refined his multiple specimen fixture to include crack growth monitoring with direct current potential drop (DCPD). Initially, his system was designed to “run out” the crack, then the test was stopped, the broken specimen removed and replaced with a new or dummy specimen, then fatiguing was resumed, generating S-N data. Eventually, DCPD was added, and additional efforts generated da/dN curves. The loading source over time was the same. A servo-hydraulic actuator was employed to test eight specimens at low frequencies, but the control method changed from load to displacement control. Results on tests that used keyhole CT (compact tension) specimens, constant K (tapered) specimens, and bend specimens have been reported. Tests were conducted in both air and sea water to add an element of corrosion fatigue.
Each of these apparatuses was designed to address very specific issues, such as those relating to materials, environmental conditions, or loading mechanisms. Although satisfactory in many respects, a need remains for systems and related methods in which multiple specimens can be concurrently evaluated and particularly subjected to cyclical loadings such as in fatigue strength testing.